Day of the Week Calculator (Any Date)
Pick any date from year 1 to year 9999 and the tool instantly tells you whether it is a Monday, Tuesday, Wednesday, Thursday, Friday, Saturday or Sunday — with leap-year flag, weekday/weekend classification, and the distance from today in days. Great for finding the day of the week of your birth, a wedding anniversary, or any historical event.
Enter a valid Gregorian date (years 1–9999).
Day of the week
Friday
Mon → Sun
Weekend?
Leap year
Formula
Uses Tomohiko Sakamoto's algorithm: h = (y + ⌊y/4⌋ − ⌊y/100⌋ + ⌊y/400⌋ + t[m−1] + d) mod 7 with t = [0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4], and y decremented by 1 when month m < 3. Result: 0 = Sunday, 1 = Monday … 6 = Saturday (re-indexed by this tool to ISO 8601 with 1 = Monday … 7 = Sunday).
- · Range: year 1 to 9999, using the proleptic Gregorian calendar — modern Gregorian rules projected backwards before its 1582 introduction. Convenient for comparing across history, but actual historical dates may have been recorded under the Julian calendar.
- · Leap-year rule: (year % 4 = 0 AND year % 100 ≠ 0) OR year % 400 = 0. So 1900 is not a leap year, but 2000 is.
- · The tool uses pure-arithmetic Sakamoto, independent of the browser Date object's time zone or clock — so the answer is identical for everyone, anywhere.
- · The "from today" count is measured at the user's local UTC midnight and updates after midnight; no hours/minutes are shown.
- · Britain (and its colonies) switched from Julian to Gregorian in September 1752, jumping straight from 1752-09-02 to 1752-09-14; Russia did not switch until 1918. When checking historical dates between 1582 and 1918, confirm which calendar the source used.
- · Sources: T. Sakamoto, comp.lang.c 1993; Zeller (1882) congruence; ISO 8601:2019 §4.2 "Representations of dates".
Frequently asked
What day of the week was I born?
Enter your birthday in Gregorian form (e.g. 1990-08-15) and the headline weekday shows immediately, along with a tiny "X days ago" line. If your birth date is before September 1752 (which essentially never happens for Hong Kong users), be aware that the original record may have been kept in the Julian calendar. This tool always uses the modern proleptic Gregorian calendar, which is the most useful basis for matching everyday history.
How accurate is Sakamoto's algorithm? Does it agree with JavaScript's Date?
Sakamoto's algorithm is pure arithmetic and effectively textbook-grade — it agrees exactly with JavaScript's Date.UTC().getUTCDay() over the entire proleptic Gregorian range. Our automated tests cross-check 200 consecutive days against the platform implementation. The benefit is that it is independent of the browser's time zone and DST, and does not even require a Date object, so the same code works in embedded systems or back-end services.
Why does the tool flag 1900 as not a leap year, but 2000 as one?
The Gregorian leap-year rule, proclaimed by Pope Gregory XIII in 1582, keeps years divisible by 4 — except those divisible by 100 unless they are also divisible by 400. 1900 ÷ 100 = 19 and is not divisible by 400, so it is not a leap year. 2000 ÷ 400 = 5, so it is. The fine-tuning keeps the civil calendar in step with the solar year (~365.2422 days); the older Julian calendar drifted by a few days per millennium.
Why does this tool put Monday as 1 when JavaScript's getDay() puts Sunday as 0?
ISO 8601 (the international date standard) starts the week on Monday and numbers the days 1 = Monday … 7 = Sunday. JavaScript's Date.getDay() follows the older US convention of 0 = Sunday … 6 = Saturday. Most of international business, banking and project management (including Hong Kong, mainland China and Europe) follows ISO 8601, so this tool uses ISO numbering — values are always in the range 1–7 with Monday as 1.
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